Microvascular endothelial cells from rat and guinea pig fat pad bound the histamine-metabolizing enzyme diamine oxidase (DAO). The extent of binding was dependent on concentration of enzyme activity and tissue. Saturation of binding sites with 5000 units DAO/ml resulted in levels of bound activity (up to 11-13 units/mg endothelial cells) in excess of that observed in all tissues except placenta. Scatchard plots suggested that there were at least two DAO binding sites (apparent Km9 and 250nM). Although the same cell preparations bound lipoprotein lipase (LPL) the presence of LPL did not interfere with binding of DAO activity except when cells were exposed to high concentrations of LPL. Alternatively, bound DAO activity was partially displaced (up to 33%) only with high concentrations (30 Mug/ml) of LPL. DAO activity may thus be bound to at least two populations of sites, one of which may bind LPL. Both enzymes, however, were displaced by heparin (0.05 to 5 units/ml) and their binding was impaired by prior treatment of cells with glycosoaminoglycan degrading enzymes. The demonstration of DAO binding to vascular endothelial cells provides a further example of the ability of these cells to bind enzymes at their surface and thereby to act on biologically active substances in the circulation.